This invention relates generally to a multi-color electrphotographic copying machine and, more particularly, to a digitally assisted, color correction system incorporated in the copying machine.
In an electrophotographic copying machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing marking particles into contact therewith. This forms a powder image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the marking particles thereto in image configuration.
Multi-color electrophotographic printing is similar to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors separation (red, green, blue) are recorded thereon. Each single color electrostatic latent image is developed with toner particles of a color complementary thereto; (cyan, magenta, yellow, respectively). This process is repeated a plurality of cycles for differently colored images and their respective complimentarily colored toner particles. Each single color toner powder image is transferred to the copy sheet in superimposed registration with the prior toner powder image. This creates a multi-layered toner powder image on the copy sheet. Thereafter, the multi-layered toner powder image is permanently affixed to the copy sheet creating a color copy.
To improve the accuracy of the color rendition, some type of color correction is usually required. Color correction is primarily required because the colored toner absorbs light of various colors which should not be absorbed e.g. magenta toner may absorb some blue light which should only be absorbed by yellow toner. One method of color correction is known from the lithographic art and is described in the "Principles of Color Reproduction" by J. A. C. Yule, John Wiley Publishers 1967 pages 41-45. One of the methods described uses a masking technique in which a positive or negative image made with one filter is combined with a negative or positive image (usually an image of the opposite sign) made with a different filter. A color correction technique known in the xerographic art is disclosed in U.S. Pat. No. 4,236,809. There an original is scanned using a conventional light lens analog scanning system while simultaneously sensing the scanned incremental image at a plurality of photosensor arrays (digital raster scanning). The raster signals are applied to a raster output scanner to expose the drum to a light image represented by the raster signals. The raster output scanner includes a laser that generates a narrow beam of light which is focused to a spot on the drum. A mirror mounted on a galvanometer sweeps the laser spot across the drum at a high speed relative to the drum peripheral movement. An optical modulator in the path of the laser beam modulates the intensity of the spot in response to the raster signals to produce a raster latent electrostatic image on the drum.
The analog and digital raster images are formed on the photoconductive drum during the same revolution and in conjunction with a screening process. The two images are in registration. The raster image is a low resolution (approximately 100 spi) correction image for the high resolution (approximately 400 spi) optical image. The resultant latent image is of a significantly enhanced quality relative to that achieved solely with the optical exposure means alone. In the case of a color copying machine, the combined optical and raster exposure means permits the machine to make good color copies of colored originals.
The color correction system disclosed in the '809 patent had several disadvantages. Three separate photosensor arrays with three associated lenses and filters are required. This leads to increased housing requirements and additional expense. Also the output of the RIS must be stored in buffers before processing the image signals to convert the raster video signals to an image correction signal. The present invention is therefore directed to an improved, low resolution, marking color correction system which incorporates a simplified RIS system which requires no buffering to provide the image correction including; a platen for supporting a colored original to be reproduced; scan illumination means mounted for movement beneath said platen; imaging means for forming successive latent images corresponding to different colors of said document on the charged surface of a photoconductive member and raster input scanning means connected to said scan illumination member and adapted for movement therewith, said RIS adapted to generate raster image signals representative of color separated images of the original document being scanned by said scan means.